There was a pause, and an eerie silence, just before he did it. A green scarf masking his face, the man held a large piece of scaffolding above his head and, surrounded by photographers, eyeballed the unprotected window of the Royal Bank of Scotland's branch on Threadneedle Street.

In that split second, one voice amid thousands in the crowd broke the silence. "Don't do it," she screamed. He did.

A bespectacled man in a beige jacket then began remonstrating with black-clad and hooded protesters. "Gandhi taught us not to use violence," said John Rowley, from the Gandhi Foundation. "This isn't violence," retorted another voice in the crowd. "We paid for this building."

Shortly after noon on an ordinary day in the Square Mile, the City's well-fed bankers would be tucking in to lunch. Yesterday, most were conspicuous by their absence as central London was convulsed by a coalition of more than 10,000 protesters pitched against global capitalism.

Organisers said they wanted to create a festival in the Square Mile, and, for a while, it looked like they would succeed. But after the noon raid on RBS the largely peaceful carnival was marred by violence between protesters and police, and ended in tragedy with the death of one man, found unconscious in a London street.

When demonstrators broke through police lines and smashed the windows of the RBS, the nationalised symbol of unregulated, boomtime greed, it appeared that the revolution, and the lynched bankers, which were predicted by Chris Knight, the suspended University of East London professor, might materialise.

Once they had broken into the bank, however, the protesters did not quite know what to do. There were no bankers to lynch (one local trader estimated there were 80% fewer City workers around than on a normal day), so the anger was vented on computers, phones and filing cabinets, which were hurled through the shattered windows.

Riot police and dogs then stormed the bank to flush out those barricaded inside after an attempt to set fire to the curtains had failed. Police also stopped an armoured vehicle; a police spokesman said that its 11 occupants were arrested for having fake police uniforms.

If they knew what they wanted, and many didn't, the demonstrators brandished an idealistic and diverse set of demands: peace in Palestine, carbon restrictions, and action against climate change. Some admitted they were as bewildered as their leaders about how to change the world for the better.

"It doesn't feel that coherent, but that's how things are at the moment. We don't know how to move forward," said Anna Coatman, 21, from Leeds.

If there was a common thread, it was a desire to stand up and be counted in opposition to the way that the leaders of the wealthiest countries proposed to fix the global recession.

"I hope it's the rebirth of the anti-globalisation movement, but they've got to work out what they want," said Billy Bragg, who performed the Internationale as 4,000 protesters converged on the Bank of England with effigies of the four horsemen of the apocalypse.

While protests in Bishopsgate retained a carnival atmosphere, the police decision to seal the G20 Meltdown march into a small grid of streets around the Bank of England until evening was the focus of a growing resentment from the crowd, which had earlier enjoyed music from solar-powered sound systems, slogans such as "Make love not leverage" and "This ruckus sponsored by Jobcentre Plus", and a brief appearance from Russell Brand, who beat a retreat when he realised that he and his three minders were a "distraction".

Later in the afternoon, student protesters sitting down in the street were charged by baton-wielding police, and complaints about the policing of the marches grew stronger. Jack Bright, 19, said: "When people surrounded RBS I could understand police tactics. But we were sat down trying to have a peaceful protest, and they started whacking us."

With his head wounds bandaged up by an ITN crew, Finn O'Sullivan, 21, said: "I just remember shields coming down on us. The police were stamping and kicking. I asked them to let me through the line for medical treatment, but they said 'no'."

In the West End, while there was a chant of "When I say banker, you say wanker", there was less tension. Marchers voiced opposition to the traditional target of US and Israeli military dominance under Stop the War coalition and CND banners.

A few defiant City workers taunted the demonstrators by waving £10 notes from distant windows. Here, at least, was a small victory for the protesters: in previous demonstrations, they had been faced with bankers brandishing £50 notes.Sporadic clashes between protesters and police continued into the evening as the crowd penned in around the Bank of England chanted: "Let us out."

Simon O'Brien, a Metropolitan police commander, said police would be asking the peaceful "flash camp" to move on before the morning.

Knight, suspended from his post at University of East London for predicting that the crowds might lynch bankers, said: "I'm feeling good. I said the revolution would happen at noon and it did. I'm sorry that the RBS got smashed, but it is only property."